Still, that's just a tiny portion of the nearly 300 million users of the world's larger social network. However, it looks as if adoption of usernames hasn't stopped. According to a recent note from Facebook engineering, it was revealed that the company has reached over 50 million users. And that milestone was passed several weeks ago - there's no telling how many people have registered a vanity URL by now

In a post entitled 'Designing the Facebook username land rush,' Facebook's Srinivas Narayanan reveals the number while explaining the difficulties of building a system capable of handling 200,000 signups in 3 minutes and planning for contingencies (aka the so-called 'nuclear option'). These are subjects we actually discussed with Facebook's Jared Morgenstern and Guy Rom at Facebook headquarters the night of the launch.

It's a testament to the success of the usernames system that 20%+ users have signed up for it within two months. At some point though, there will be a ceiling where the registration of new usernames turns into a trickle. We're curious how many users will have vanity URLs by then.